Dynamic response of lightweight bridges and control strategies to reduce deck accelerations

While for standard footbridge only a few vibration modes are within the critical footfall frequency range and hove to be considered for the dynamic design, lightweight structures display multiple modes in that frequency range. Furthermore the modal masses are very little so also the smaller load components for the excitation of higher harmonic vibration modes can become significant to assess the vibration susceptibility. Various observations have been made during experimental tests of these structures and will be introduced in this contribution using the example of a canopy walk structure and a stress ribbon bridge. To understand the observed effects at such continuous dynamic systems for pedestrian loading, numerical calculations have been performed for which a similar lightweight structure has been modelled and pedestrian loading was simulated with several footfall frequencies. Similar to the experimentally investigated structures it was found that the multimodal dynamic response of the bridges strongly depends on the pedestrian loading (footfall frequency). Experimental tests have shown, that the application of a Tuned Mass Damper System to dampen only the critical mode that is within the footfall frequency range is not sufficient to reduce the multimodal dynamic response of the lightweight bridge structures under pedestrian so the occurring accelerations remain below certain comfort levels. For example, it was observed that while a TMD system was enabled for 1 relevant mode that was significant for the un-dampened structure, the bridge then responded with similar vibration levels in an adjacent vibration mode. Also it occurred that the dynamic response in vibration modes that were in range of sub-harmonics or higher harmonics caused vibrations above comfort level, due to the relatively small modal mass, so the load component even with smaller Fourier coefficients was sufficient to cause discomfort.